Journal of Hazardous Materials 261 (2013) 295– 306

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Journal of Hazardous Materials

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Nanoscale zero-valent iron supported on mesoporous silica:Characterization and reactivity for Cr(VI) removal from aqueoussolution

Eleni Petalaa,b, Konstantinos Dimosa, Alexios Douvalis c, Thomas Bakasc, Jiri Tucekb,Radek Zboril b, Michael A. Karakassidesa,∗

a Department of Materials Science and Engineering, University of Ioannina, GR-45110 Ioannina, Greeceb Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University, 17.listopadu 1192/12,771 46 Olomouc, Czech Republicc Department of Physics, University of Ioannina, GR-45110 Ioannina, Greece

h i g h l i g h t s

• New nanohybrid exhibits magneticand molecular sieves properties.

• Material shows well dispersed andstabilized ZVI nanoparticles on inor-ganic support.

• Material demonstrates high Cr(VI)chemical reduction efficiency.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 March 2013Received in revised form 18 July 2013Accepted 21 July 2013Available online xxx

Keywords:Nanoscale zero-valent ironMesoporous silicaCr(VI) remediationCharacterizationKinetics

a b s t r a c t

MCM-41-supported nanoscale zero-valent iron (nZVI) was sytnhesized by impregnating the mesoporoussilica martix with ferric chloride, followed by chemical reduction with NaHB4. The samples were studiedwith a combination of characterization techniques such as powder X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) and Mössbauer spectroscopy, N2 adsorption measurements, transmissionelectron microscopy (TEM), magnetization measurements, and thermal analysis methods. The experi-mental data revealed development of nanoscale zero-valent iron particles with an elliptical shape and amaximum size of ∼80 nm, which were randomly distributed and immobilized on the mesoporous silicasurface. Surface area measurements showed that the porous MCM-41 host matrix maintains its hexag-onal mesoporous order structure and exhibits a considerable high surface area (609 m2/g). Mössbauerand magnetization measurements confirmed the presence of core–shell iron nanoparticles composed ofa ferromagnetic metallic core and an oxide/hydroxide shell. The kinetic studies demonstrated a rapidremoval of Cr(VI) ions from the aqueous solutions in the presence of these stabilized nZVI particles onMCM-41, and a considerably increased reduction capacity per unit mass of material in comparison tothat of unsupported nZVI. The results also indicate a highly pH-dependent reduction efficiency of thematerial, whereas their kinetics was described by a pseudo-first order kinetic model.

© 2013 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +30 26510 07276; fax: +30 26510 07074.E-mail address: mkarakas@cc.uoi.gr (M.A. Karakassides).

1. Introduction

Since 1991, zero-valent iron (Fe0 or ZVI) is being used as areactive material for remediation of contaminated sites due to itsgreat ability to directly or indirectly reduce different types of toxic

0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jhazmat.2013.07.046

296 E. Petala et al. / Journal of Hazardous Materials 261 (2013) 295– 306

compounds and metal ions such as As(III), Pb(II), Cu(II), Ni(II), Co(II)and Cr(VI) [1–5]. In the environment, iron is always found in itsthermodynamically stable oxide forms as under normal conditions,zero-valent iron is unstable and gets oxidized very easily. This factalso explains why metallic iron particles exhibit a high capability inconverting various contaminants to less toxic forms [6]. The path-ways for contaminant reduction involve: (i) direct reduction at thesurface of ZVI, (ii) catalytic reduction through Fe(II) at the surfaceof corrosion products or (iii) adsorption onto corrosion products[7].

It is well-known that zero-valent iron particle have a core–shellarchitecture [1,3]. The core consists of metallic iron (�-Fe) whileoxides and hydroxides form the shell as a result of oxidation ofthe iron surface. High activity and efficiency of nZVI particles comefrom combination of components that form their structure. Coreis responsible for the reduction of the contaminants as its oxida-tion in an aqueous environment generates appropriate reactantssuch as Fe(II), H/H2 and iron hydroxides and oxides [8]. However,the presence of the iron oxides causes passivation of the core sur-face, although in some cases a contaminant should migrate acrossthe film to adsorb on the Fe0 surface and undergo reduction. Alter-natively, the oxide layer should be electronic conductive to allowelectron transfer [8]. The sorption and precipitation of the cationsdepend on some factors, such as the kind of cation, the pH valueand concentration [5].

Zero-valent iron can react with oxygen and water followingclassical electrochemical corrosion reactions in which iron giveselectrons to water and Fe(II) gives electrons to O2 [9]. In addi-tion, ions like Cr(VI) can readily accept the electrons and reduceto lower oxidation states [10,11]. Chromium is a widely usedheavy metal in many engineering and chemical industries suchas electroplating facilities and tannery industries [12]. It existsmainly in the oxidation states of Cr(VI) and Cr(III) [13]. Hexavalentchromium is considered to be one of the most highly toxic environ-mental contaminants. Cr(VI) compounds are known to be highlytoxic, carcinogenic and mutagenic. They can cause several seri-ous health problems like diseases striking the respiratory tract,male reproductive system, stomach and small intestine. Reduc-tion to Cr(III) followed by precipitation of chromium species maybe considered a satisfactory solution in eliminating the toxicity ofCr(VI) because the toxicity of trivalent chromium toward a liv-ing cell is 500–1000 times less than hexavalent chromium [14].However, exposure to excessive doses of Cr(III) for long peri-ods of time may also cause some adverse health effects [15–17].Therefore, wastewater treatment systems based only on Cr(VI)reduction at acidic pH values, i.e., necessary for high chromiumreduction efficiency [18], cannot remove Cr(III) from the solu-tion, since resulting chromium exhibits high solubility at pH < 4.Moreover, Fe0 even though in the form of scrap iron has theadvantage not only to reduce Cr(VI)–Cr(III) at pH ≥ 3 but also toremove the reduced hexavalent chromium from the wastewa-ter as it forms insoluble mixed Cr–Fe (oxy)hydroxide phases viaits corrosion [18–20]. In addition, the iron corrosion producesiron oxides in which a part of Cr(VI) can also be adsorbed[21].

Zero-valent iron particles, synthesized in the nanoscale form,demonstrate high surface-to-volume ratios, and, thus, high sur-face activity and increased gravimetric uptake capacity, thoughttheir depletion especially in acidic conditions [7,22]. However,most synthetic ZVI nanoparticles procedures display a signifi-cant disadvantage, i.e., the observed aggregation in time, whichfinally decreases the activity of iron nanoparticles. For this rea-son, the modification and/or stabilization of ZVI nanoparticles insolid matrices followed by their good dispersion which can lead tosteady or even enhanced remediation ability, is deemed necessary.In the past few years, a large number of research works have been

emerged focusing on production of more active and stable nZVI par-ticles, simplification of the synthetic procedure and reduction of thecost, stimulating thus their utilization in large-scale applications[23–26].

Among the various heavy metals, Cr(VI) exhibits high sensi-tivity upon reduction by ZVI [21]. As recently reported [27], theremoval efficiency of zero-valent iron ranked from top to bottom:As > Cr > Cu > Hg > Pb > Zn > Cd > Ni, whereas especially Cr(VI) canreact with ZVI by more than one mechanism, i.e., reduction, adsorp-tion, co-precipitation. Therefore, Cr(VI) is a suitable candidate forthe evaluation of zero valent iron nanoparticles in environmentalapplications [21].

Aggregation phenomena are reported to be considerablyreduced when a matrix or a supporting material is used for thereduction of an iron salt and, additionally, for the production, thehomogeneous dispersion, and hosting of nZVI; ZVI nanoparticlessynthesized in this way tend to exhibit a narrower size distribu-tion. Therefore, supported nZVI show a higher activity comparedto non-supported systems [6,28,29]. The synthesis of nanoscalezero-valent iron has been achieved in the presence of various sup-port materials such as inorganic compounds and clay minerals[30–36], polymers [6,37–39] or other organic compounds [40–42],nanostuctured carbons [43–45], and silica-based molecular sieves[46–49].

Among the support materials, silica-based molecular sieveswith uniform pore sizes, such as zeolites, MCM-41 and SBA-15, areparticularly interesting as host matrices for loading ZVI nanopar-ticles, since their nanoporous matrices may additionally affect thecatalytic and sorption properties of the hosted nanoparticles. Pre-vious attempts to disperse nanoparticles on silica mesoporousmaterials were limited to the hexagonal SBA-15 phase. The MCM-41 solid (Mobil Composition of Matter No. 41), a member ofthe M41S family exhibits regular hexagonal arrays of cylinder-like pores and a variable pore diameter between 1.5 and 20 nm,high crystallinity, and high thermal stability [50,51]. Its large sur-face area (∼1000 m2/g) makes it ideal for catalytic applicationsas well as its loading with metal cations [52]. In the presentwork, we report on the synthesis and stabilization of nZVI in thepores of the MCM-41 matrix. The final materials were tested fortheir applicability in the remediation of Cr(VI). The main goalsof this work were: (a) successful production of stable nZVI witha high gravimetric ratio in the hybrids, (b) high activity of thefinal materials concerning the reduction of Cr(VI), (c) develop-ment of new composite nanomaterials endowed with improvedproperties through synergetic effects, and (d) materials whichcan be easily separated and collected by an external magneticfield.

2. Materials and methods

2.1. Materials

The following reagents were used as purchased withoutfurther purification: tetraethylorthosilicate (TEOS) 98% fromSigma–Aldrich (131903), aqueous ammonia solution (NH3)25 wt% from Fluka (09860), cetyltrimethylammonium bromide(CTAB) 95% from Sigma–Aldrich (855820), ethanol (EtOH) 99.5%from Panreac (121086.1212), hydrochloric acid (HCl) 1 N fromRiedel de Haën (35328), analytical grade acetone 99.95% fromFisher Scientific (A/0600/17), phosphoric acid (H3PO4) 85%from Sigma–Aldrich (215104), potassium dichromate (K2Cr2O7)from Sigma–Aldrich (P2588), 1,5-diphenylcarbazide from Aldrich(42860), sodium borohydride (NaBH4) from Merck (8.06373) andiron (III) chloride hexahydrate (FeCl3·6H2O) from Sigma–Aldrich(207926).

E. Petala et al. / Journal of Hazardous Materials 261 (2013) 295– 306 297

2.2. Synthesis of MCM-41

The MCM-41 sample was synthesized by hydrolyzing 50 g ofTEOS, added to one liter polyethylene bottle containing 417.5 g ofH2O, 268.5 g of NH3 (25 wt%) and 10.5 g of CTAB. The resulting mix-ture was stirred for 30 min. The product was retrieved after heattreatment at 80 ◦C for 96 h. It was filtered, rinsed with cold EtOH andfinally placed on a plate for air-drying (sample: MCM-41). The tem-plate was removed by calcination at 550 ◦C for 5 h with a 2 ◦C/minheating rate.

2.3. Synthesis of nZVI

One gram of FeCl3·6H2O was dissolved in 50 mL of EtOH. Thereducing agent was prepared by dissolving 1 g of NaBH4 in 50 mLof deionized water. The borohydride solution was added drop-wiseto the ferric solution while swirling by hand. Black agglomeratedparticles appeared immediately indicating the presence of zero-valent iron. After complete addition of the borohydride solution, themixture was left to settle for 15 min. The mixture was centrifugatedand the separated particles were washed with EtOH three timesand dried in vacuum (sample: nZVI). Experimental parameters suchas reaction time, reactant concentrations and volumes were keptconstant in order to produce samples with consistent properties.

2.4. Preparation of nZVI supported on MCM-41

The formation of the nZVI onto the inner surface of MCM-41was carried out by wet impregnation technique. 1.5 g of ferric chlo-ride (FeCl3·6H2O) was dissolved in 10 mL of absolute ethanol andwas dropped on 0.7 g of the mesoporous material. The mixturewas heated at ∼80 ◦C up to the evaporation of the solvent andthe obtained powder was dispersed in 100 mL of absolute ethanol.The reducing agent was prepared by dissolving 1.5 g of NaBH4in 150 mL of deionized water and was added drop-wise to theMCM-41-Fe3+ solution while swirling by hand. After the completeaddition of the borohydride solution, the mixture was left to settlefor 15 min. The mixture was centrifugated and the separated par-ticles were washed with absolute ethanol three times and dried invacuum (sample: nZVI@MCM-41). In this case, a lower concentra-tion of the reactive agent was used in order to avoid destructionof the MCM-41 structure. Dissolution of MCM-41 framework canbe caused by a slow reaction between sodium borohydride andethanol.

2.5. Characterization

The �–2� X-ray powder diffraction data (XRD) were collectedwith a D8 Advance Bruker diffractometer using Cu K (40 kV,40 mA, � = 1.54178 A) radiation. Diffraction patterns were collectedin the 2� range from 1.5◦ to 80◦, in steps of 0.02◦ and 2Ys countingtime per step. Pellets of pulverized samples dispersed in KBr wereused for recording Fourier transform Infrared spectra on a PerkinElmer GX Fourier transform spectrometer in the frequency rangeof 400–4000 cm−1. The reported spectra are an average of 64 scansat 2 cm−1 resolution. Thermo-gravimetric analysis (TG) and differ-ential thermal analysis (DTA) were performed using a Perkin ElmerPyris Diamond TG/DTA instrument. Samples of ∼4 mg were heatedin not-flowing atmospheric air from 25 to 750 ◦C with a 5 ◦C/minheating rate.

The nitrogen adsorption–desorption isotherms were measuredat 77 K on a Sorptomatic 1990, thermo Finnigan porosime-ter. Specific surface areas (SBET) were determined with theBrunauer–Emmett–Teller (BET) method using adsorption datapoints in the relative pressure P/P0 range from 0.01 to 0.30. Sur-face areas (St) were also determined from t-plots which were

constructed using nitrogen adsorption data with the de Boerstandard. Mesoporous surface area (Smeso) and volume (Vmeso)were also determined from Va–t-plots. Total pore volume (Vpore)was estimated from the adsorbed amount at a relative pressureof 0.99. The desorption branches of the isotherms were used forthe pore size calculations according to the Kelvin equation, i.e.,rk ∼ 4.146/(log(P0/P)) (Å), where P0 is the saturated vapour pres-sure in equilibrium with the adsorbate condensed in a capillary ora pore, P is the vapour pressure of a liquid contained in a cylindricalcapillary, and rk is the Kelvin radius of the capillary or pore. TheKelvin equation was used according to the BJH method for calcula-tion of core radii and the pore size distribution (PSD) of the samples.All samples used for the surface analyses were outgassed at 150 ◦Cfor 16 h under high vacuum (10−5 mbar) before the measurements.

Transmission electron microscopy (TEM) observations wereperformed on a JEOL JEM-2010 transmission electron microscopeequipped by a LaB6 cathode (accelerating voltage of 200 kV; point-to-point resolution of 0.194 nm). A drop of high-purity distilledwater, containing the ultrasonically dispersed particles, was placedonto a Holey Carbon film supported by a copper-mesh TEM grid andair-dried at room temperature.

57Fe Mössbauer spectra were collected in transmission geome-try at 77 K, using a constant-acceleration spectrometer, equippedwith a 57Co(Rh) source kept at room temperature (RT) and a liq-uid N2 bath Mössbauer cryostat (Oxford Instruments). Velocitycalibration of the spectrometer was carried out using metallic �-Fe at RT and all isomer shift (ı) values are given relative to thisstandard. Magnetization (M) measurements were performed ona superconducting quantum interference device (SQUID) magne-tometer (Quantum Design, MPMS XL-7). The hysteresis loops werecollected at temperatures of 5 and 300 K in external applied mag-netic fields (B) ranging from −7 to +7 T. The zero-field-cooled(ZFC) and field-cooled (FC) magnetization curves were recordedon warming the samples in the temperature range from 5 to 300 Kunder external magnetic fields of 0.01 and 0.1 T after cooling inzero magnetic field and on cooling under the same fields, respec-tively.

2.6. Batch experiments for the reduction of Cr(VI)

In order to test the reduction capacity of nZVI and MCM-41-supported nZVI, appropriate amount of each sample with the sameiron content (18 mg of nZVI@MCM-41 and 3.5 mg nZVI) was addedto 100 mL solution of K2Cr2O7 in distilled water (concentrationof Cr(VI): 6 mg/L) previously stirred at room temperature for 12 hin a closed vessel. Tests were performed at two different pH val-ues (pH = 3 and pH = 5) and various reaction times. The pH of thesuspensions was adjusted to 5 or 3 by adding hydrochloric acidsolution (1 N). During the reaction, at different periods, a vol-ume of the under study suspension (5 mL) was withdrawn andcentrifuged. From the resulting supernatant Cr(VI) solution a vol-ume of 0.3 mL was used for colorimetric measurements, while thesolid and the rest unused centrifuged solution were returned tothe suspension. The Cr(VI) concentration of the solution was thendetermined by using the 1,5-diphenylcarbazide method [53]. The1,5-diphenylcarbazide method is based on the reaction of Cr(VI)cations with 1,5-diphenylcarbazide molecules leading to the for-mation of a red-purple chromium 1,5-diphenylcarbazide complex.Diphenylcarbazide solution was prepared by dissolving 0.025 g of1.5-diphenylcarbazide in 10 mL of analytical grade acetone. Thesolution was stored in a closed flask at 4 ◦C up to 7 days. Phosphoricacid solution was prepared by diluting 0.5 mL of concentrated phos-phoric acid in 10 mL of deionized water. To each tested solution(0.3 mL), 160 �L of the diphenylcarbazide solution and 80 �L ofthe phosphoric acid solution were added (total volume adjustedat 3 mL with 2.460 mL H2O). The solution was swirled and left

298 E. Petala et al. / Journal of Hazardous Materials 261 (2013) 295– 306

Fig. 1. X-ray diffraction patterns of MCM-41 (a), supported zero-valent ironnanoparticles on mesoporous silica, nZVI@MCM-41 (b), and unsupported nZVI (c).Inset shows the marked part of XRD pattern of nZVI@MCM-41 in magnification.

for 10 min to allow color development and then the solution’sconcentration was determined spectrophotometrically by usingUV–vis spectroscopy at 540 nm using a calibration curve. Absorp-tion spectra of solutions were measured using a 10 mm optical pathquartz vesicle. UV–vis spectra were recorded on a Shimadzu UV-2401PC two beam spectrophotometer in the range of 350–700 nm,at a step of 0.5 nm, using combination of deuterium and halogenlamps as sources. The calibration curve was initially determinedby using the absorbance spectra of standard chromium solutions(Supporting Information Figs. S1 and S2). The total chromium con-centration was determined by oxidizing any trivalent chromiumwith potassium permanganate [18,54], followed by analysis ashexavalent chromium. Trivalent chromium was determined fromthe difference between total and hexavalent chromium. In addition,because pH has a strong effect on Cr(VI) sorption and reduction,the pH value of the suspension was also measured at differentperiods.

2.7. Adsorption isotherms

Adsorption isotherms of Cr(VI) on MCM-41 were determinedusing batch equilibrium of 0.145 g of dosage materials in 100 mLCr(VI) solution at various initial concentrations. All experimentswere varied out in a stealing-steel cooling bath whereas the reac-tion mixture was kept under suspension using a magnetic stirrerlocated under the bath base. During the experiments the solutiontemperature was kept constant at 25 ◦C by means of cooling water.After equilibrium, the solution was centrifuged and filtered withfilter paper while the supernatant was measured by the diphenyl-carbazide method. The adsorbed amount was calculated from thedifference between initial (C0) and equilibrium (Ce) concentrationaccording to the equation qe = [(C0 − Ce) × V]/m, where V is the totalvolume of the solution (mL) and m is the mass of adsorbent (g) [55].

The variation of pH during the reaction of Cr(VI) was also mea-sured versus time. Appropriate amount of each sample (14.5 mgfor MCM-41, 18 mg for nZVI@MCM-41, and 3.5 mg for nZVI) wasadded to 100 mL Cr(VI) 6 mg/L solution and pH was measured untilit reached a plateau.

3. Results and discussion

3.1. Structural and textural characterization

Fig. 1 shows the X-ray diffraction pattern from a powder ofnZVI and that of the same nanoparticles supported on mesoporous

Fig. 2. FT-infrared spectra of MCM-41 (a) and nZVI@MCM-41 (b) sample.

silica. As it is evident, the XRD pattern of the nZVI@MCM-41sample displays three reflection peaks, typical of the MCM-41mesoporous materials. They arise from the quasi-regular arrange-ment of the mesopores in the bulk material. Specifically, they showa characteristic strong reflection at low scattering 2� angles, corre-sponding to a d1 0 0 spacing at about 40 A. Furthermore, the patternof nZVI@MCM-41 sample clearly displays the higher order 110 and200 reflections suggesting that the hexagonal structure of the sup-port matrix remains unimpaired upon the nanoparticle growth onits surfaces [56]. Besides, the characteristic peak at 2� = 44.9◦ in theXRD patterns of nZVI and nZVI@MCM-41 sample indicates the pres-ence of ion predominantly in the Fe0 state, as reflections due to ironoxides such as magnetite, �-Fe2O3 or hematite (2� ∼ 36◦) were notobserved in both patterns.

The infrared spectra of MCM-41 and nZVI@MCM-41 samplesare shown in Fig. 2. Both spectra (a and b) display bands at 1235,1075, 800 and 460 cm−1. Bands at similar frequencies in the spectraof crystalline and amorphous SiO2 have been assigned to char-acteristic vibrations of Si O Si bridges cross-linking the silicatenetwork [57–60]. Thus, the stronger bands at 1235 at 1075 cm−1

can be assigned to the asymmetric stretching mode vibrations ofthe Si O Si group whereas the appearance of the two maximacan be related to the LO and TO components of its vibration [61].The bending motion of oxygen in the same bridge is responsi-ble for the band at 800 cm−1 (motion along the bisector of theSi O Si bridging group) while the rocking motion of bridging oxy-gens perpendicular to the Si O Si plane can be correlated withthe 460 cm−1 band. On the other hand, only the spectrum of MCM-41 sample exhibits the band at 960 cm−1 which can be assignedto vibrations of Si O− and Si OH bonds in the silicate tetrahedralunits. In the spectrum of nZVI@MCM-41, a significant reductionof absorption in this spectral area can be attributed to the forma-tion of Si O− Fe bonds. Moreover, the effect of NaBH4 on infraredvibrations of silanols groups was found to be negligible under theused conditions whereas the broad absorption band at 1390 cm−1

observed in the spectrum of nZVI@MCM-41 can be correlated withthe presence of borate species produced during the iron nanopar-ticle synthesis.

Fig. 3 shows the DTA and TG curves recorded under air for theMCM-41 matrix before (a) and after the nZVI upgrowth (b). Thepristine MCM-41 matrix exhibits a smooth DTA curve without anyremarkable exothermic or endothermic peak and a weight loss lessthan 0.5% as the mesoporous silica was treated at 550 ◦C for 5 hat its last step of synthesis. On the other hand, the TGA curve ofnZVI@MCM-41 shows a weight loss of ∼7Ywt% at low tempera-tures (up to 150 ◦C) followed by a weight increase of ∼4Ywt% in the

E. Petala et al. / Journal of Hazardous Materials 261 (2013) 295– 306 299

Fig. 3. DTA/TG diagrams of MCM-41 (a) and nZVI@MCM-41 (b) sample.

temperature range of 200 to 500 ◦C. The ∼7Ywt% loss at low tem-peratures is assigned to ethanol and water desorption [62], whilethe ∼4Ywt% increase of the sample could be attributed to nZVI oxi-dation which were anchored on the silicate matrix. The observedmass increase is also related to a weak exothermic peak observedat 400 ◦C in the DTA curve of nZVI@MCM-41 sample, implying thenZVI oxidation. This assignment is in accordance with the recentstudy on thermal oxidation of iron nanoparticles [6,63]. Further-more, taking into account the ∼4% weight increase in the TGAsignal and the fact that at temperatures >200 ◦C the thermal oxida-tion of Fe0 in air gives Fe2O3, the total zero-valent iron-content ofnZVI@MCM-41 was determined at about ∼9.3% of its total mass.

The surface area and the pore structure of MCM-41 andnZVI@MCM-41 samples were determined from the nitrogenisotherms analysis. As it is shown in Fig. 4, both samples exhibittype-IV isotherms, characteristic of mesoporous materials. Thepresence of a sharp sorption step in these adsorption curves, nearto a 0.3 value of P/P0, indicates that the two solids possess a welldefined array of regular mesopores. Based on these data, the specificsurface area (SBET) for the MCM-41 pristine sample was calcu-lated to be 1060 m2/g, the mean pore diameter 2.6 nm, and thetotal pore volume 0.79 cm3/g (Table 1). On the other hand, thenZVI@MCM-41 sample, which was prepared using the above MCM-41 as a matrix, displayed similar type-IV adsorption/desorptionisotherms but showed a lower surface area of ∼609 m2/g, meanpore diameter of ∼2.4 nm and a total pore volume of 0.32 cm3/g,which provides evidence that its pores are filled with zero-valent

Fig. 4. Nitrogen adsorption–desorption isotherms for MCM-41 (a) and nZVI@MCM-41 (b) sample. Inset: pore size distributions calculated from nitrogen desorptionbranch for MCM-41 (c) and nZVI@MCM-41 (d) sample.

iron nanoparticles. Alternatively, these data suggest that the poreaccess of nZVI@MCM-41 is limited because of the formation of Fe0

nanoparticles with sizes larger than 3 nm on the external surfacesof the matrix.

Fig. 5 shows the TEM images of iron nanoparticles supportedon the MCM-41 matrix (a and b) and that of as-prepared Fe0

nanoparticles (c) after reduction of FeCl3 in aqueous solution byNaBH4 for comparison purposes. It is evident that nanoparticles,elliptical in shape and with a maximum size of ∼80 nm, wererandomly distributed and immobilized on the mesoporous silicasurface. These nanoparticles seem to possess a core–shell structure(Fig. 5b) according to the core–shell model of nZVI [1] in whichthe core is made up mainly of Fe0 and the shell is largely of ironoxides/hydroxides origin, formed from the oxidation of zero-valentiron. TEM analysis of image (b) supports this view (number 1 and2 indicate the regions of nanoparticles which are attributed to Fe0

core and oxide shell, respectively). In addition, the linear array ofmesoporous silica channels is clearly observed in the region of num-ber 3 of Fig. 5b, indicating, in agreement with the XRD results, thatthe sample keeps the hexagonal mesoporous order structure ofMCM-41 support. It should be noted that the degree of aggrega-tion of these particles is limited in contrary to that exhibited byZVI nanoparticles synthesized without any support (see Fig. 5c forcomparison).

From TEM images it can also be calculated the Fe0 content inthe nZVI particles equal to 47.82Ywt% (calculations in SupportingInformation for Fig. S3), which is a value in good agreement with lit-erature [55,64]. The above percentage, combined with the TGA data,also suggests that the total nZVI wt% content of the hybrid mate-rial (nZVI@MCM-41) is 19.45% (as the iron of the shell is alreadyoxidized and would not attribute to the observed weight increasein the TGA signal) which means that the same iron content for thebatch experiments is 180 mg/L for the hybrid material and 35 mg/Lfor the nZVI.

300 E. Petala et al. / Journal of Hazardous Materials 261 (2013) 295– 306

Table 1Textural properties of MCM-41 and nZVI@MCM-41 sample.

Sample SBET (m2/g)a St (m2/g)b Smeso (m2/g)b Vpore (cm3/g)c Vmeso (cm3/g)b dBJH (nm)d

MCM-41 1060 1052 1040 0.79 0.72 2.6nZVI@MCM-41 609 612 592 0.39 0.32 2.4

a Total specific surface area (SBET) form multi-point BET analysis.b Total surface area (St), mesoporous surface area (Smeso) and mesoporous volume (Vmeso) determined form t-plots (standard: de Boer).c Total pore volume (Vpore) estimated from the adsorbed amount at relative pressure of 0.99.d Mean pore diameters (dBJH) determined form BJH analysis of the desorption data.

3.2. Magnetic properties and Mössbauer study of pristine andtreated samples with Cr6+ solutions

57Fe Mössbauer spectra of nZVI@MCM-41 sample as well assamples after treatment with aquatic Cr6+ solution and after treat-ment in water (for 24 h), recorded at 77 K, are shown in Fig. 6.The spectrum of the initial sample shows magnetically split andquadrupole split contributions. A model of five independent spec-tral components was used to fit this spectrum and the resultingMössbauer hyperfine parameters are listed in Table 2. In particular,the resonant lines of the central magnetically split contributionsseem to show a superposition of a major part with broad resonantlines (component 1) and a minor part with narrow resonant lines(component 3). The ı and hyperfine magnetic field (Bhf) values ofthese two components correspond to iron atoms having metalliccharacteristics. These components represent the iron atoms situ-ated at the metallic cores of the ZVI nanoparticles. The differencesbetween the corresponding ı and Bhf values and the line-width ofthese two components reflect the difference in the sizes of the ZVInanoparticles. The smaller nanoparticles, which are represented bycomponent 1, show superparamagnetic relaxation effects, as theyare drastically influenced by thermal agitation due to their reducedsizes [65]. On the other hand, the larger nanoparticles, representedhere by component 3, possess hyperfine parameters resemblingthose of bulk metallic iron [66] due to their increased sizes.

The main quadrupole split central part of the spectrum is fit-ted by component 2. The hyperfine parameters of this componentcorrespond to a Fe3+ in a high-spin (S = 5/2) state and representferric ions in oxide or hydroxide phases [67]. These phases consti-tute most probably the shell of the nanoparticles, which is formedduring their preparation or by oxidation when the nanoparticlesget in contact with atmospheric oxygen and moisture. The para-magnetic (only quadrupole split) state of this components suggests

that the shell of the nanoparticles is composed mainly of ironoxide/hydroxide such as ferrihydrite which may have magneticordering temperatures lower than 77 K [67] or these shell phasesshow superparamagnetic phenomena due to their reduced dimen-sions (thin shells).

A minor broad magnetically split contribution is also evidentby its two outer higher-velocity resonant lines near ±8 mm/s(component 4), which emerge from the background. Its ı valuecorresponds to Fe3+ in a high-spin (S = 5/2) state, and its high Bhfvalue (see Table 2) suggests that it represents such ions in ironoxide/hydroxide phase constituting most probably the shell of thelarger ZVI nanoparticles, whose core is represented in the spectrumby component 2. Finally, a minor quadrupole split contribution(component 5) with Fe2+ high-spin (S = 2) state characteristics (seeTable 2) was introduced to the fitting model in order to cover themissing absorption intensities near −0.5 and +2.5 mm/s. This com-ponent corresponds to partially oxidized iron ions, most probablyin some minor impurity iron oxide phases produced during thepreparation procedure.

The spectrum of the nZVI@MCM-41 after treatment with aquaticCr6+ solution sample (Cr6+-reacted sample) resembles that of thecorresponding initial sample and is fitted using the same model.However, the comparison between the resulting absorption areasof the components used to fit the two spectra reveals importantdifferences that reflect the effect of the redox reaction between theCr6+ ions in the solution and the nZVI. In particular, component 1 inthe Mössbauer spectrum of the Cr6+-reacted sample which repre-sents the core of the nZVI, looses absorption area, while component3, which represents their shell, gains absorption area, relative to theareas found for the corresponding components of the spectrum ofthe as-made sample (see Table 2). All the other components in thespectrum of the Cr6+-reacted sample retain essentially the same rel-ative absorption areas they had in the as-made sample. This result

Fig. 5. TEM images of nZVI@MCM-41 (a and b) and nZVI (c) sample.

E. Petala et al. / Journal of Hazardous Materials 261 (2013) 295– 306 301

Table 2Values of the hyperfine parameters derived from the best fits of the hybrid nZVI@MCM-41 as-made, Cr6+-reacted and water-treated samples, recorded at 77 K, where ı is theisomer shift (relative to �-Fe at RT), �/2 is the half width at half maximum, �EQ is the quadrupole splitting, 2ε is the quadrupole shift, Bhf is the hyperfine magnetic field,�Bhf is the spreading of Bhf and A is the relative spectral absorption area for each component. The components discussed in the text and their assignments are also listed.Typical errors are ±0.02 mm/s for ı, �/2, �EQ and 2ε and ±5% for A.

Sample Component ı (mm/s) �/2 (mm/s) �EQ or 2ε (mm/s) Bhf (T) �Bhf (T) A (%) Assignment

As-made 1 0.23 0.29 −0.04 27.9 6.3 62 Fe0 in core (small ZVI)2 0.47 0.30 0.81 – – 23 Fe3+ in shell (small ZVI)3 0.12 0.17 −0.02 33.8 0.4 6 Fe0 in core (large ZVI)4 0.47 0.18 0.00 50.8 4.3 6 Fe3+ in shell (large ZVI)5 1.12 0.35 2.91 – – 3 Fe2+ in impurities

Cr6+-reacted 1 0.26 0.29 −0.05 28.1 6.5 51 Fe0 in core (small ZVI)2 0.47 0.30 0.80 – – 34 Fe3+ in shell (small ZVI)3 0.11 0.18 −0.03 33.6 0.6 5 Fe0 in core (large ZVI)4 0.47 0.18 0.00 49.0 4.3 6 Fe3+ in shell (large ZVI)5 1.12 0.35 2.79 – – 4 Fe2+ in impurities

Water-reacted A 0.47 0.29 0.79 – – 85 Fe3+ as in shellB 0.47 0.16 −0.01 31.6 8.9 15 Fe3+ as in shell

is a direct proof that during the redox reaction, Cr6+ ions diffusingthrough the ZVI nanoparticles shell are reduced to Cr3+ by oxidizingthe metallic core Fe0 atoms to Fe3+ ions, increasing thus the shellthickness. On the other hand, the large ZVI nanoparticles (repre-sented by components 3 and 4) stay almost inert in this reaction.

Fig. 6. 57Fe Mössbauer spectra of the hybrid nZVI@MCM-41 as-made, Cr6+-reactedand water-treated samples recorded at 77 K. The points correspond to the exper-imental data and the continuous lines to the components used to fit the spectra.The numbers and letters correspond to the components discussed in the text andTable 2.

Thus, the second important conclusion is that the size of nZVI is asignificant factor for the yield of the reduction of Cr6+ in the solu-tion – the smaller ZVI particles are simultaneously the most activeto this procedure as well.

Comparison of the former two spectra with the spectrumrecorded after treating the as-made sample with water revealsadditional information on the dynamics of the redox reaction. Thisspectrum is composed of two components, one major paramagneticdoublet (component A) and one minor very broad magnetic sextet(component B), whose values of the hyperfine parameters are char-acteristic of Fe3+ ions only and resemble much those of components2 and 4 used to fit the spectra discussed above (see Table 2). Thisresult indicates that degradation of the metallic Fe0 phase duringthe hydrolysis reaction causes all iron atoms to be fully oxidizedto Fe3+, transforming completely the core of the ZVI nanoparti-cles to the oxide/hydroxide phases of the shell through oxygen andwater molecule diffusion. However, it is important to note that theappearance of characteristic Fe0 components in the spectrum ofthe Cr6+-reacted sample denotes a preservation of the Fe0 cores.This suggests that in the case of the reaction of ZVI nanoparticleswith Cr6+ ions in the corresponding aquatic solution, diffusion ofCr ions in the shell of the nanoparticles forms a shell with an iron-chromium oxide/hydroxide composition, most probably situatednear the core that prevents the remaining Fe0 core from further oxi-dation. On the other hand, it can be accompanied with a saturationeffect, suppressing the ability of these modified ZVI nanoparticlesto reduce additional Cr6+ ions existing in the solution.

The physical properties of nZVI@MCM-41 sample can berevealed from its magnetization measurements. Fig. 7 shows thehysteresis loops of this sample recorded at 300 and 5 K. Ferromag-netic response with maximum magnetizations (at 7 T) of ∼36.8 and∼43.7 Am2/kg at 300 and 5 K, respectively, are observed, while sat-uration is not achieved at both temperatures, as a non-vanishingdM/dB slope is evident at the high applied magnetic field region.In addition, this slope seems to be higher for the measurement at5 K compared with that observed at 300 K. Moreover, hysteresisbehavior is clearly seen in the insets of Fig. 7, with the values of thecoercive fields of ∼54.7 and ∼73.5 mT at 300 and 5 K, respectively.The saturation magnetization of pure bulk metallic iron varies from∼218.0 to ∼221.9 Am2/kg between 300 and 5 K, respectively, whileits coercivity lies in the range of 1.0–2.0 mT [68]. Consequently, theloop characteristics of the nZVI@MCM-41 sample, observed both at300 and 5 K, reflect the size effect of the ZVI particles, as the non-vanishing high field dM/dB slope, the decrease in the saturationmagnetization and the increase in the coercive field are identi-fying features of their nanostructured nature. However, the largedecrease in the saturation magnetization could be attributed also

302 E. Petala et al. / Journal of Hazardous Materials 261 (2013) 295– 306

Fig. 7. Hysteresis loops of nZVI@MCM-41 sample recorded at 300 K (a) and 5 K (b).

to the presence of the additional masses of the MCM matrix, as wellas of the oxide/hydroxide shell phases in the sample, which providediamagnetic and antiferromagnetic contributions, respectively.

The ZFC/FC magnetization curves of the nZVI@MCM-41 sample,depicted in Fig. 8, confirm the conclusions derived from the hystere-sis loops measurements. In the ZFC magnetization branch, recordedunder applied magnetic fields of 0.01 and 0.1 T, the magnetizationvalues show a steep increase with increasing temperature, followedby a more gradual increase above about 50 K. On the contrary, FCmagnetization curves display a relative stable increase in the mag-netization values with decreasing temperature which is followedby a steep increase at temperatures below 10 K. This is a typicalbehavior found for such core–shell nanoparticles composed of aferromagnetic metallic core and an oxide/hydroxide shell [67]. Inaddition, this indicates that the ZVI nanoparticles in the studiedsystem appear in a range of sizes that restrain them from adoptingcomplete superparamagnetic characteristics.

3.3. Cr(VI) reduction

The capacity of the nZVI@MCM-41 sample on hexavalentchromium removal was estimated by conducting a series of batchexperiments. Fig. 9a displays the removal kinetics at 6 mg/L of

Fig. 8. ZFC and FC magnetization curves of the nZVI@MCM-41 sample recorded at0.01 (a) and 0.1 T (b).

initial concentration in Cr(VI) of the aqueous solution as a func-tion of reaction time for 180 mg/L dosage and two different pHvalues (3 and 5). The reaction capacity of the pristine MCM-41 isalso shown in the same figure for comparison purpose. The kineticsof the removal process was simulated using the pseudo first-ordermodel [69] expressed by the equation:

dC/dt = −kobs · C (1)

where C is the concentration of contaminant (i.e., Cr(VI)) in theaqueous phase (mg/L) and kobs is the observed rate constant of thepseudo first-order reaction (h−1). According to this model, kobs isthe product of KSA·�m·as where KSA is the surface-area normalizedrate coefficient (L/h m2), �m is the mass concentration (g/L) and as

is the specific surface area (m2/g) of the used material respectively.Since KSA, �m, and as are constant for a specific reaction, their prod-uct can be expressed with only one parameter, i.e., kobs. Integrationof Eq. (1) gives:

ln(

C

C0

)= −kobs · t (2)

where C0 = [Cr(VI)]0 and C = [Cr(VI)]t are Cr(VI) concentrations inthe solution after 0 and t hours of reaction and kobs is the observedrate constant of the pseudo first-order reaction (h−1). The plots ofln([Cr(VI)]t/[Cr(VI)]0) versus time produced linear lines (see Fig. 9b)

E. Petala et al. / Journal of Hazardous Materials 261 (2013) 295– 306 303

Fig. 9. (a) Effect of initial pH value (5 and 3) on the Cr(VI) removal efficiency bypristine MCM-41 and nZVI@MCM-41 sample and (b) pseudo-first-order kineticsfor reduction of Cr(VI) by nZVI@MCM-41 (nZVI@MCM-41 dose was 180 mg/L andinitial Cr(VI) concentration was 6 ppm). (c) Cr(VI) removal efficiency of nZVI@MCM-41 sample (dosage of 18 mg containing ∼3.5 mg of nZVI) in comparison to that ofunsupported nZVI (same dosage of 3.5 mg nZVI).

with correlation coefficients (R2) reaching a value more than 0.97,indicating that the rate of chromium removal fitted well this model.The kobs values are equal to the slope of the line by plottingln([Cr(VI)]t/[Cr(VI)]0) versus time; kobs were found to be 1.02 and0.332 h−1 for pH = 3 and 5, respectively. The Gibbs free energy (�G0)was also calculated for nZVI@MCM-41 hybrid at room temperatureat pH = 3 and 5 using the equations KD = qe/Ce and �G◦ = −R·T·lnKD

where Ce is the equilibrium concentration (mg/mL), qe is theamount of Cr(VI) removed at equilibrium (mg/g), KD is the dis-tribution coefficient (mL/g), R is the gas constant (8.134 J/mol K)and T is the temperature (K) [70]. Employing this equation, wearrived at −28.91 kJ/mol (pH = 3) and −20.77 kJ/mol (pH = 5). Fromthese negative values, one can conclude that the removal processis spontaneous and thermodynamically favorable.

It is well known that the removal capacity and mechanism ofzero-valent iron nanoparticles is strongly affected by the pH. AtpH = 3, Cr(VI) was completely eliminated within 3 h; however, athigher pH values, passivation of the surface of iron particles prob-ably occurred, limiting the further removal capability. While thezero-valent iron particles undergo oxidation, iron oxide/hydroxidespecies are formed on the surface of the hybrid material. The sol-ubility of these oxides/hydroxides is not favorable at high valuesof pH while their formation is related also with the initial con-centration of the Cr(VI). Generally, it has been reported that thecapacity and the reaction rate of ZVI depend on the total surfacearea, the type of contaminant, the initial concentration of ironand the pollutant, the presence of other oxidants such as oxy-gen and nitrate, and on solution chemistry such as solution pH[71].

Besides, Cr(VI) can be reduced by Fe(II) or atomic/molecularhydrogen (H/H2), which is derived from the oxidation of zero valentiron, and by electrons originating directly from the Fe0 core, ifits oxide layer shell is conductive [72], since the outer oxide filmon the ZVI particles restricts the accessibility between iron coreand Cr(VI) species. On the other hand, as the Fe(II) cations dif-fuse more favorably through the porous oxide film of ZVI towardto the contaminant species [8], it can be suggested that the ironnanoparticles probably operate as generators of reducing species(Fe(II), H/H2) and removing agents (Fe hydroxides and oxides) thanas direct reducing agents. Despite Fe(III) and Cr(III) that are pro-duced in these reactions, co-precipitation of Cr(III)/Fe(III) in theform of CrxFe1−x(OH)3 or CrxFe1−x(OOH), x < 1 is also possible. ThepH is a critical criterion in determining the thickness of the dou-ble layer at the interface between the oxide surface and solution[7]. Following these results, one can conclude that for the lowerpH values, an increased reactivity is observed while the electrontransfer is promoted and the passivation of the iron particles ishindered.

It should be noted that in all kinetic experiments, the hexa-valent chromium reduction proceeded rapidly in the first 2 hwhile in the next hours, it was decelerated progressively. Thelower pH values prevail in efficiency as it could be observedwhen the initial pH value of the solutions increases, the removalpercentages of Cr(VI) decrease. Specifically, the efficiency of thenZVI@MCM-41 sample for the chromium removal after 9 h reac-tion time at pH = 3 was determined to be 100% for 180 mg/L while64% was found for 100 mg/L of material dose (its kinetic is notshown in the figure). In addition, concerning the parent MCM-41material, only at pH = 3, a small removal of Cr(VI) occurred, i.e.,∼6%.

Fig. 9c shows the effectiveness of the nZVI@MCM-41 sample incomparison to unsupported nZVI as a function of reaction time atpH = 3 and [Cr(VI)]0 = 6 mg/L. In order to be comparable, the dose ofthe unsupported nZVI was chosen to be equal with the Fe0 loadingof the hybrid material (∼9.3Ywt%), as it was calculated accordingto the TGA results. It should be noted that, bared nZVI exhibit asignificant slower and lower reaction rate (0.10 h−1) with efficiencydetermined to be only 26% after 3 h of treatment while for the sametime, the nZVI@MCM-41 material has removed 95% of Cr(VI) dueto its higher reaction rate (1.02 h−1).

Along these lines, the variation of pH during the reactions ofCr(VI) with nZVI and nZVI@MCM-41 at pHinitial = 3 and 5 was deter-mined and the results are presented in Fig. 10 (MCM-41 effect

304 E. Petala et al. / Journal of Hazardous Materials 261 (2013) 295– 306

Fig. 10. Variation of pH during the reaction process of Cr(VI) by nZVI@MCM-41 andnZVI at initial pH values 3 and 5.

on pH in Supporting Information Fig. S4). As shown in this fig-ure, the solution pH of nZVI@MCM-41 system originally increasedto higher pH values (3.6 and 7.7 from pHinitial 3 and 5, respec-tively) and reached a plateau with increasing the reaction time.These pH changes are similar to that of the nZVI “standalone” sys-tem. However, the use of MCM-41 as support material resulted toeven higher pH values than those observed in the single nZVI treat-ment (3.2 and 6.7). These results are in agreement with those ofa recently published work on degradation of trinitroglycerin usingZVI nanoparticles supported on SBA-15 [47]. These features are alsocommonly observed in the leading edge of zero-valent iron perme-able reactive barriers (PRBs) and are due to abrupt depletion ofprotons during the ZVI oxidation. [73]. Normally, this described pHincrease leads to the development of a passivation layer on the ZVIsurface that can prevent the transport of Cr(VI) species to the under-lying iron surface and, thus, their subsequent reduction. In previousstudies concerning the mechanism of Cr(VI) reduction using ZVIsupported on pillared bentonite or silica [55,73] an opposite pHtrend has been observed during the same reaction and it has beenassigned to buffering effect of aluminol or silanol groups on the sup-port surfaces. According to that mechanism, as the pH increasedwith protons being consumed, the aluminol or silanol groups ofsupport material, dissociated to compensate the depletion of pro-tons. Thus, the generated protons maintained the pH close to theinitial value, which in turn reduced the surface passivation of iron.In addition, negatively charged surface sites rendered by release ofprotons from aluminol or silanol groups may have served as collec-tors for iron cations, increasing the probability of co-precipitationof reaction products. In fact, as it has been reported, the bentoniteand silica surface can strongly bind Fe(III) and Cr(III) via surfacecomplexation [73–75].

On the other hand, considering the fact that mesoporous silicasupport of nZVI@MCM-41 has a pHpzc equal to 3.2 [50] and as thesolution initial pH (3 or 5) increases rapidly during the reaction(Fig. 10) to higher values, the MCM-41 surface will bear a progres-sively increasing negative charge, so the adsorption of Cr(VI) onit will be limited (Supporting Information Fig. S5). In contrast, atpH > 3.2 the MCM-41 negatively charged surface can act as scav-enger but can enhance the reaction rate through its interaction withreaction precipitates (Fe3+, Cr3+).

In other words, MCM-41 could react as a scavenger for theprecipitates and in that way, minimizing the agglomeration ofthe reduced products on the surface of nZVI, providing morefree area for reaction, facilitated their well dispersion and non-agglomeration as well. This leads to a higher surface area and higherreactivity of zero-valent iron nanoparticles which are stabilized on

the MCM-41 surface, exhibiting, thus, an improved efficiency forhexavalent chromium removal from aqueous solutions. In addi-tion, the experimental results showed also a progressive increaseof pH with the time that suggests a slower rate for nZVI pas-sivation in nZVI@MCM-41 material in comparison to that forunsupported nZVI. Besides, the aqueous Cr(III) concentrations inthe solutions after the reaction with nZVI@MCM-41 were cal-culated and found to be zero and 2 mg/L at pH values 5 and3 respectively [54]. Considering, that the used initial amount ofCr(VI) was 6 mg/L and the removal efficiency of nZVI@MCM-41was determined to be 100% (pH = 3) and 53% (pH = 5) it is obviousthat 4 mg/L and 3.2 mg/L of insoluble species of Cr(III) precipi-tated on the surface of MCM-41 respectively. It can be reasonablyassumed that the higher pH values observed during the reaction(Fig. 10) provided favorable conditions for the precipitation ofCr(III)/Fe(III) oxides/hydroxides on the high surface area of MCM-41matrix.

4. Conclusions

Nanoscale zero-valent iron supported on surfaces of meso-porous MCM-41 silica have been prepared, characterized, andtested for Cr(VI) treatment. The synthesis method was based onborohydride reduction and use of a ferric chloride impregnatingsilica mesoporous matrix of the well known MCM-41 material.This preparation method leads to well dispersed and stabilizednanoscale zero-valent iron particles (nZVI) on a porous matrixwhich retains its ordered structural characteristics and exhibitsa considerably high surface area (∼600 m2/g). The as-synthesizednanoparticles seems to possess a typical core–shell architecture ofnZVI in which the core is consisted mainly of Fe0 and the shell islargely of iron oxides/hydroxides origin formed from the oxida-tion of zero-valent iron. Transmission electron microscopy imagesshowed the presence of homogeneously dispersed nZVI parti-cles, elliptical in shape and with almost uniform sizes (60–80 nm),thought Mössbauer spectroscopy data also suggest the existenceof smaller nanoparticles, which show superparamagnetic behaviordue to their reduced sizes. In addition, the Mössbauer spectra ofthe samples treated with aquatic Cr(VI) solutions revealed that thesize of nZVI is a significant factor for the yield of the reduction ofchromium in the solution, as the smaller ZVI particles are simulta-neously the most active to this procedure as well. Magnetizationmeasurements showed that nZVI@MCM-41 sample exhibits ferro-magnetic behavior with maximum magnetizations (at 7 T) of ∼36.8and 43.7 Am2/kg at 300 and 5 K, respectively. The kinetic data arewell fitted with a pseudo-first-order reaction model in order to ade-quately describe the kinetic uptake of Cr(VI) on nZVI@MCM-41. Theresults revealed that the synthesized material exhibits enhancedefficiency on Cr(VI) removal in comparison with nZVI particles,indicating a synergetic effect between reduction by nZVI and MCM-41 inducing precipitate formation of the reduced species. Thehosting within the mesoporous silica matrix prevented agglom-eration of the nanoscale iron and thus, increased its surface areaallowing accessibility of the Cr(VI) species to the nZVI particles.

Acknowledgements

The authors acknowledge the support by the OperationalProgram Research and Development for Innovations – Euro-pean Regional Development Fund (CZ.1.05/2.1.00/03.0058) andOperational Program Education for Competitiveness – Euro-pean Social Fund (CZ.1.07/2.3.00/20.0017, CZ.1.07/2.3.00/20.0170,CZ.1.07/2.3.00/20.0155, and CZ.1.07/2.3.00/20.0056) of the Min-istry of Education, Youth and Sports of the Czech Republic. We alsothank Dr. Safarova for TEM observations.

E. Petala et al. / Journal of Hazardous Materials 261 (2013) 295– 306 305

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013.07.046.

References

[1] X.Q. Li, D.W. Elliott, W.X. Zhang, Zero-valent iron nanoparticles for abatement ofenvironmental pollutants, Critical Reviews in Solid State and Materials Sciences31 (2006) 111–122.

[2] C. Noubactep, A critical review on the process of contaminant removal inFe0–H2O systems, Environmental Technology 29 (2008) 909–920.

[3] Y.P. Sun, X.Q. Li, J. Cao, W.X. Zhang, H.P. Wang, Characterization of zero-valentiron nanoparticles, Advances in Colloid and Interface Science 120 (2006) 47–56.

[4] W.X. Zhang, Nanoscale iron particles for environmental remediation: anoverview, Journal of Nanoparticle Research 5 (2003) 323–332.

[5] C . Üzüm, T. Shahwan, A.E. Eroglu, I. Lieberwirth, T.B. Scott, K.R. Hallam, Appli-cation of zero-valent iron nanoparticles for the removal of aqueous Co2+ ionsunder various experimental conditions, Chemical Engineering Journal 144(2008) 213–220.

[6] S.M. Ponder, J.G. Darab, J. Bucher, D. Caulder, I. Craig, L. Davis, N. Edelstein,W. Lukens, H. Nitsche, L. Rao, D.K. Shuh, T.E. Mallouk, Surface chemistry andelectrochemistry of supported zerovalent iron nanoparticles in the remediationof aqueous metal contaminants, Chemistry of Materials 13 (2001) 479–486.

[7] C. Noubactep, S. Caré, R.A. Crane, Nanoscale metallic iron for environmen-tal remediation: prospects and limitations, Water, Air, and Soil Pollution 223(2012) 1363–1382.

[8] C. Noubactep, An analysis of the evolution of reactive species in Fe0/H2O sys-tems, Journal of Hazardous Materials 168 (2009) 1626–1631.

[9] M. Stratmann, J. Müller, The mechanism of the oxygen reduction on rust-covered metal substrates, Corrosion Science 36 (1994) 327–359.

[10] R.A. Crane, T.B. Scott, Nanoscale zero-valent iron: future prospects foran emerging water treatment technology, Journal of Hazardous Materials211–212 (211) (2012) 112–120.

[11] A. Fiúza, A. Silva, G. Carvalho, A.V. de la Fuente, C. Delerue-Matos, Heteroge-neous kinetics of the reduction of chromium (VI) by elemental iron, Journal ofHazardous Materials 175 (2010) 1042–1047.

[12] M. Muthukrishnan, B.K. Guha, Effect of pH on rejection of hexavalent chromiumby nanofiltration, Desalination 219 (2008) 171–178.

[13] C.H. Weng, Y.T. Lin, T.Y. Lin, C.M. Kao, Enhancement of electrokinetic reme-diation of hyper-Cr(VI) contaminated clay by zero-valent iron, Journal ofHazardous Materials 149 (2007) 292–302.

[14] M. Costa, Potential hazards of hexavalent chromate in our drinking water,Toxicology and Applied Pharmacology 188 (2003) 1–5.

[15] J. Cerulli, D.W. Grabe, I. Gauthier, M. Malone, M.D. McGoldrick, Chromiumpicolinate toxicity, Annals of Pharmacotherapy 32 (1998) 428–431.

[16] M.D. Stearns, M.S. Silveira, K.K. Wolf, Chromium(III) tris(picolinate) is muta-genic at the hypoxanthine (guanine) phosphoribosyltransferase locus inChinese hamster ovary cells, Mutation Research 513 (2002) 135–142.

[17] S.A. Kareus, C. Kelley, H.S. Walton, P.R. Sinclair, Release of Cr(III) from Cr(III)picolinate upon metabolic activation, Journal of Hazardous Materials B84(2001) 163–174.

[18] M. Gheju, A. Iovi, I. Balcu, Hexavalent chromium reduction with scrap iron incontinuous-flow system. Part 1: effect of feed solution pH, Journal of HazardousMaterials 153 (2008) 655–662.

[19] M. Gheju, I. Balcu, Hexavalent chromium reduction with scrap iron incontinuous-flow system. Part 2: effect of scrap iron shape and size, Journalof Hazardous Materials 182 (2010) 484–493.

[20] M. Gheju, I. Balcu, Removal of chromium from Cr(VI) polluted wastewatersby reduction with scrap iron and subsequent precipitation of resulted cations,Journal of Hazardous Materials 196 (2011) 131–138.

[21] D. O’Carroll, B. Sleep, M. Krol, H. Boparai, C. Kocur, Nanoscale zero valent ironand bimetallic particles for contaminated site remediation, Advances in WaterResources 51 (2013) 104–122.

[22] M.M. Scherer, S. Richter, R.L. Valentine, P.J.J. Alvarez, Chemistry, Microbiologyof permeable reactive barriers for in situ groundwater clean up, Critical Reviewsin Microbiology 26 (2000) 221–264.

[23] Y.P. Sun, X.Q. Li, W.X. Zhang, H.P. Wang, A method for the preparation of stabledispersion of zero-valent iron nanoparticles, Colloids and Surfaces A 308 (2007)60–66.

[24] A. Tiraferri, K.L. Chen, R. Sethi, M. Elimelech, Reduced aggregation and sedimen-tation of zero-valent iron nanoparticles in the presence of guar gum, Journal ofColloid and Interface Science 324 (2008) 71–79.

[25] K. Siskova, J. Tucek, L. MacHala, E. Otyepkova, Air-stable nZVI formationmediated by glutamic acid: solid-state storable material exhibiting 2D chainmorphology and high reactivity in aqueous environment, Journal of Nanopar-ticle Research 14 (2012) 1–13.

[26] R. Zboril, M. Andrle, F. Oplustil, L. Machala, J. Tucek, J. Filip, Z. Marusak, V.K.Sharma, Treatment of chemical warfare agents by zero-valent iron nanopar-ticles and ferrate(VI)/(III) composite, Journal of Hazardous Materials 211–212(2012) 126–130.

[27] S.-C. Tsai, Heavy Metal Removal From Water by Zero-valent Iron, Departmentand Graduate Institute of Environmental Engineering and Management, 2012(Master’s Thesis).

[28] H. Zhang, Z.H. Jin, L. Han, C.H. Qin, Synthesis of nanoscale zero-valent iron sup-ported on exfoliated graphite for removal of nitrate, Transactions of NonferrousMetals Society of China 16 (2006) s345–s349.

[29] L. Li, M. Fan, R. Brown, J. Van Leeuwen, J. Wang, W. Wang, Y. Song, P. Zhang,Synthesis, properties and environmental applications of nanoscale iron-basedmaterials: a review, Critical Reviews in Environmental Science and Technology36 (2006) 405–431.

[30] C . Üzüm, T. Shahwan, A.E. Eroglu, K.R. Hallam, T.B. Scott, I. Lieberwirth, Synthe-sis and characterization of kaolinite-supported zero-valent iron nanoparticlesand their application for the removal of aqueous Cu2+ and Co2+ ions, AppliedClay Science 43 (2009) 172–181.

[31] P. Wu, S. Li, L. Ju, N. Zhu, J. Wu, P. Li, Z. Dang, Mechanism of the reduction of hexa-valent chromium by organo-montmorillonite supported iron nanoparticles,Journal of Hazardous Materials 219–220 (2012) 283–288.

[32] X. Qiu, Z. Fang, B. Liang, F. Gu, Z. Xu, Degradation of decabromodiphenyl ether bynano zero-valent iron immobilized in mesoporous silica microspheres, Journalof Hazardous Materials 193 (2011) 70–81.

[33] Y. Li, Z. Jin, T. Li, S. Li, Removal of hexavalent chromium in soil and ground-water by supported nano zero-valent iron on silica fume, Water Science andTechnology 63 (2011) 2781–2787.

[34] J. Zhan, T. Zheng, G. Piringer, C. Day, G.L. McPherson, Y. Lu, K. Papadopoulos, V.T.John, Transport characteristics of nanoscale functional zerovalent iron/silicacomposites for in situ remediation of trichloroethylene, Environmental Scienceand Technology 42 (2008) 8871–8876.

[35] Y. Zhang, Y. Li, J. Li, G. Sheng, Y. Zhang, X. Zheng, Enhanced Cr(VI) removal byusing the mixture of pillared bentonite and zero-valent iron, Chemical Engi-neering Journal 185–186 (2012) 243–249.

[36] Y. Zhang, Y. Li, J. Li, L. Hu, X. Zheng, Enhanced removal of nitrate by a novelcomposite: nanoscale zero valent iron supported on pillared clay, ChemicalEngineering Journal 171 (2011) 526–531.

[37] H. Kim, H.J. Hong, Y.J. Lee, H.J. Shin, J.W. Yang, Degradation of trichloroethyleneby zero-valent iron immobilized in cationic exchange membrane, Desalination223 (2008) 212–220.

[38] D.E. Meyer, K. Wood, L.G. Bachas, D. Bhattacharyya, Degradation of chlori-nated organics by membrane-immobilized nanosized metals, EnvironmentalProgress 23 (2004) 232–242.

[39] H.-Y. Shu, M.-C. Chang, C.-C. Chen, P.-E. Chen, Using resin supported nano zero-valent iron particles for decoloration of Acid Blue 113 azo dye solution, Journalof Hazardous Materials 184 (2010) 499–505.

[40] H. Kim, H.-J. Hong, J. Jung, S.-H. Kim, J.-W. Yang, Degradation of trichloroethy-lene (TCE) by nanoscale zero-valent iron (nZVI) immobilized in alginate bead,Journal of Hazardous Materials 176 (2010) 1038–1043.

[41] T. Liu, L. Zhao, D. Sun, X. Tan, Entrapment of nanoscale zero-valent iron inchitosan beads for hexavalent chromium removal from wastewater, Journal ofHazardous Materials 184 (2010) 724–730.

[42] V. Madhavi, A.V.B. Reddy, K.G. Reddy, G. Madhavi, A simple method for thedetermination of efficiency of stabilized Fe0 nanoparticles for detoxification ofchromium (VI) in water, Journal of Chemical and Pharmaceutical Research 4(2012) 1539–1545.

[43] H. Zhu, Y. Jia, X. Wu, H. Wang, Removal of arsenic from water by supportednano zero-valent iron on activated carbon, Journal of Hazardous Materials 172(2009) 1591–1596.

[44] F.J. Zhang, H. Xuan, F.Z. Xie, T. Jiang, T. Chen, W.C. Oh, Enhancing decolorizationfor methyl orange in aqueous solution by graphene supported nanoscale zerovalent iron, Asian Journal of Chemistry 24 (2012) 3994–3996.

[45] X. Ling, J. Li, W. Zhu, Y. Zhu, X. Sun, J. Shen, W. Han, L. Wang, Synthesisof nanoscale zero-valent iron/ordered mesoporous carbon for adsorp-tion and synergistic reduction of nitrobenzene, Chemosphere 87 (2012)655–660.

[46] J. Li, H. Li, Y. Zhu, Y. Hao, X. Sun, L. Wang, Dual roles of amphiphilic triblockcopolymer P123 in synthesis of �-Fe nanoparticle/ordered mesoporous silicacomposites, Applied Surface Science 258 (2011) 657–661.

[47] R. Saad, S. Thiboutot, G. Ampleman, W. Dashan, J. Hawari, Degradation oftrinitroglycerin (TNG) using zero-valent iron nanoparticles/nanosilica SBA-15composite (ZVINs/SBA-15), Chemosphere 81 (2010) 853–858.

[48] P. Zhang, X. Tao, Z. Li, R.S. Bowman, Enhanced perchloroethylene reductionin column systems using surfactant-modified zeolite/zero-valent iron pellets,Environmental Science and Technology 36 (2002) 3597–3603.

[49] T. Zheng, J. Zhan, J. He, C. Day, Y. Lu, G.L. McPherson, G. Piringer, V.T. John,Reactivity characteristics of nanoscale zerovalent iron-silica composites fortrichloroethylene remediation, Environmental Science and Technology 42(2008) 4494–4499.

[50] K. Dimos, P. Stathi, M.A. Karakassides, Y. Deligiannakis, Synthesis and character-ization of hybrid MCM-41 materials for heavy metal adsorption, Microporousand Mesoporous Materials 126 (2009) 65–71.

[51] P. Stathi, K. Dimos, M.A. Karakassides, Y. Deligiannakis, Mechanism of heavymetal uptake by a hybrid MCM-41 material: surface complexation and EPRspectroscopic study, Journal of Colloid and Interface Science 343 (2010)374–380.

[52] A.B. Bourlinos, A. Simopoulos, N. Boukos, D. Petridis, Magnetic modificationof the external surfaces in the MCM-41 porous silica: synthesis, character-ization, and functionalization, Journal of Physical Chemistry B 105 (2001)7432–7437.

306 E. Petala et al. / Journal of Hazardous Materials 261 (2013) 295– 306

[53] D.J. Fyfe, Rapid, automated and cost-effective analyses of hexavalent chromiumfor WEEE/RoHS using the Varian Cary 50 UV–Vis Microplate Reader System,Varian Inc. Application Note 00533.

[54] APHA, AWWA, WEF, Standard Methods for the Examination of Water andWastewater, 21st ed., United Book Press, Inc., Baltimore, MD, 2005.

[55] Y. Li, J. Li, Y. Zhang, Mechanism insights into enhanced Cr(VI) removal usingnanoscale zerovalent iron supported on the pillared bentonite by macroscopicand spectroscopic studies, Journal of Hazardous Materials 227–228 (2012)211–218.

[56] K. Dimos, L. Jankovic, I.B. Koutselas, M.A. Karakassides, R. Zboril, P. Komadel,Low-temperature synthesis and characterization of gallium nitride quantumdots in ordered mesoporous silica, Journal of Physical Chemistry C 116 (2012)1185–1194.

[57] C.T. Kirk, Quantitative analysis of the effect of disorder-induced mode couplingon infrared absorption in silica, Physical Review B 38 (1988) 1255–1273.

[58] P.G. Pai, S.S. Chao, Y. Takagi, G. Lucovsky, Infrared spectroscopic study of SiOx

films produced by plasma enhanced chemical vapor deposition, Journal of Vac-uum Science and Technology A 4 (1986) 689–694.

[59] M.R. Almeida, C.G. Pantano, Structural investigation of silica gelfilms by infrared spectroscopy, Journal of Applied Physics 68 (1990)4225–4232.

[60] E.I. Kamitsos, A.P. Patsis, G. Kordas, Infrared-reflectance spectra of heat-treatedsol–gel-derived silica, Physical Review B 48 (1993) 12499–12505.

[61] K. Dimos, M.K. Antoniou, A. Meichanetzoglou, S. Lymperopoulou, M.-D.Ouzouni, I.B. Koutselas, D. Fokas, M.A. Karakassides, R.G. Agostino, D. Gour-nis, Naphthalene-based periodic nanoporous organosilicas: I. Synthesis andstructural characterization, Microporous and Mesoporous Materials 158 (2012)324–331.

[62] A. Jitianu, M. Crisan, A. Meghea, I. Rau, M. Zaharescu, Influence of the silicabased matrix on the formation of iron oxide nanoparticles in the Fe2O3–SiO2

system, obtained by sol–gel method, Journal of Materials Chemistry 12 (2002)1401–1407.

[63] D. Wen, P. Song, K. Zhang, J. Qian, Thermal oxidation of iron nanoparticles and itsimplication for chemical-looping combustion, Journal of Chemical Technologyand Biotechnology 86 (2011) 375–380.

[64] Y. Li, Y. Zhang, J. Li, X. Zheng, Enhanced removal of pentachlorophenol by anovel composite: nanoscale zero valent iron immobilized on organobentonite,Environmental Pollution 159 (2011) 3744–3749.

[65] S. Mørup, Mössbauer effect in small particles, Hyperfine Interactions 60 (1990)959–974.

[66] N.N. Greenwood, T.C. Gibb, Mössbauer Spectroscopy, Chapman and Hall Ltd.,London, 1971.

[67] A.P. Douvalis, R. Zboril, A.B. Bourlinos, J. Tucek, S. Spyridi, T. Bakas, A facilesynthetic route toward air-stable magnetic nanoalloys with Fe-Ni/Fe-Co coreand iron oxide shell, Journal of Nanoparticle Research 14 (2012), 1130(1–16).

[68] B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, Wiley, NJ, 2009.[69] L.-N. Shi, X. Zhang, Z.-L. Chen, Removal of Chromium (VI) from wastewa-

ter using bentonite-supported nanoscale zero-valent iron, Water Research 45(2011) 886–892.

[70] M. Baikousi, A.B. Bourlinos, A. Douvalis, T. Bakas, D.F. Anagnostopoulos, J. Tucek,K. Safárová, R. Zboril, M.A. Karakassides, Synthesis and characterization of �-Fe2O3/carbon hybrids and their application in removal of hexavalent chromiumions from aqueous solutions, Langmuir 28 (2012) 3918–3930.

[71] X.-Q. Li, J. Cao, W.-X. Zhang, Stoichiometry of Cr(VI) immobilization usingnanoscale zerovalent iron (nZVI): a study with high-resolution X-rayphotoelectron spectroscopy (HR-XPS), Industrial and Engineering ChemistryResearch 47 (2008) 2131–2139.

[72] M. Odziemkowski, Spectroscopic studies and reactions of corrosion prod-ucts at surfaces and electrodes, Spectroscopic Properties of Inorganic andOrganometallic Compounds 40 (2009) 385–449.

[73] Y.J. Oh, H. Song, W.S. Shin, S.J. Choi, Y.-H. Kim, Effect of amorphous silica andsilica sand on removal of chromium(VI) by zero-valent iron, Chemosphere 66(2007) 858–865.

[74] S.E. Fendorf, G.M. Lamble, M.G. Stapleton, M.J. Kelley, D.L. Sparks, Mechanismsof chromium(III) sorption on silica. 1. Chromium(III) surface structure derivedby extended X-ray absorption fine structure spectroscopy, Environmental Sci-ence and Technology 28 (1994) 284–289.

[75] K. Flogeac, E. Guillon, M. Aplincourt, Adsorption of several metal ions onto amodel soil sample: equilibrium and EPR studies, Journal of Colloid and InterfaceScience 286 (2005) 596–601.